In recent years, with the development of mobile communication devices and mobile electronic equipments, the demand for power sources thereof have been greatly increased. Since batteries, especially lithium secondary batteries that can be repeatedly charged/discharged, have high electromotive force and high energy density so as to be repeatedly used, they have been in extensive use as power sources of mobile electronic equipments and the like.
With the miniaturization as well as size-reduction of mobile electronic equipments, however, the demand is increasingly high for batteries with higher energy density, and the emergence of new electrode materials with higher energy density than conventional materials have been desired. In this background, active efforts are underway to develop new electrode materials with higher energy density that would directly lead to an increase in energy densities of batteries.
In order to fabricate batteries with higher energy density and lighter weight, studies have recently been conducted on the use of organic compounds as electrode materials. Organic compounds are so light as to have a specific gravity of about 1 g/cm3, which is lighter than lithium cobaltate currently in use as a material for lithium secondary batteries. The use of organic compounds as electrode materials therefore allows fabrication of batteries lighter in weight and higher in capacity than conventional batteries.
For example, U.S. Pat. No. 5,833,048 and Japanese Patent No. 2,715,778 (Japanese Laid-Open Patent Publication No. Hei 5-074459) have proposed lithium secondary batteries where an organosulfur compound having a disulfide bond is used as an electrode material. Such an organosulfur compound is most simply represented by: M+—−S—R—S-M+. Herein, R represents an aliphatic group or an aromatic group, S represents sulfur, and M+ represents a proton or a metal cation. The above compounds are bonded to each other via the S—S bond through an electrochemical oxidative reaction to give a polymer with a structure of M+-S—R—S—S—R—S—S—R—S−-M+. The polymer thus produced returns to the original monomers through an electrochemical reduction reaction. In lithium secondary batteries, this reaction is applied to the charging/discharging reaction in secondary batteries.
Further, U.S. patent application Ser. No. 5,523,179 has proposed the use of elemental sulfur as an electrode material.
In either case, however, the problem arises that the materials have low cycle life characteristics although it is possible to achieve high capacity. This is because a recombination frequency is low in the dissociation and recombination of a disulfide bond during the oxidation-reduction reaction of a sulfur-based material. Low recombination frequency means that all reactive portions cannot react even if the material theoretically has high energy density. Therefore, it cannot actually be said that the compounds of the above related art examples are materials having high energy density.
As thus described, in lightweight and high energy-density electrochemical devices using the organosulfur compound having the disulfide site as the electrode reaction site, as an electrode material, the organosulur compound structurally varies remarkably through an oxidation-reduction reaction, raising a problem of a low cycle characteristic. In the light of this, an object of the present invention is to improve a cycle characteristic of a lightweight electrochemical device having high energy density.